1. Field of the Invention
The present inventions relates to a coordinate measuring apparatus using a ball probe such as manual operation type three-dimensional measuring apparatus, a CNC three-dimensional measuring apparatus having a manual teaching function, a digitizer and so on. More particularly, this invention relates to a ball probe and a measuring apparatus using the ball probe to be used advantageously for measuring objects to be measured of particularly thin plate form, protruded portions of an object to be measured or the like.
2. Prior Art
In a manual operated three-dimensional measuring apparatus using a ball probe having a ball for being brought into contact with an object to be measured on the tip of a probe, for example, the contact position of the ball to the object can be determined by manually moving the ball probe in respective directions in a three-dimensional space, and by measuring the central coordinate value of the ball at the instant when the ball comes into contact with the object to be measured or during their contact, and then by performing the offset treatment of the radius of the ball to the central coordinate value of the ball. However, the direction of the offset to the measured central coordinate value of the ball should be beforehand specified or determined by geometric calculation using a plurality of measured central coordinate values of the ball. For instance, when the object to be measured is a hole, the ball shall be guided to three different points in the inside of this hole and brought into contact to measure respective central coordinate values. Then, the circle adapting calculation shall be performed to these three coordinate values to obtain the center and the radius of the circle, and the radius of the hole can be determined by adding this radius value and the ball radius value.
In case of thin objects, such as sheet metal wherein the thickness of the object to be measured is smaller than the ball radius, the operation of guiding a probe to bring the ball apex exactly into contact with the object to be measured for measuring the hole as mentioned above is difficult and requires skill. For instance, as shown in FIG. 9, a significant measurement error may occur when a probe 100 comes into contact with an object 101 to be measured such as thin sheet metal at a wrong measuring position B in place of a right measuring position A to be brought into contact.
Conventionally, to solve the problem mentioned above, a cylindrical probe 110 as shown in FIG. 10A has been used in place of a ball probe. By using this cylindrical probe 110, a correct measurement can be assured wherever any positions on the outer circumferential surface of the probe 110 comes into contact with an object 101 to be measured, provided that the central axis of the probe 110 and the object 101 to be measured intersect at right angles. However, one drawback of this measurement using this cylindrical probe 110 is that the vertical contact position of the probe 110 and the object 101 to be measured can not be determined. The cylindrical probe 110 is inappropriate for measuring, for example, objects to be measured having partially pointed portions or the like, because it can not identify the position in the vertical direction. As a consequence, for instance, objects to be measured having thin configuration and ordinary configuration in a mixed manner are extremely troublesome as the probe should be exchanged on the halfway of measurement and, moreover, a displacement error may often occur due to the probe exchange. As shown in Fig.10B, another problem is that a significant measurement error may occur if the central axis of the cylindrical probe 110 and the object 101 to be measured having thin sheet metal configuration are not orthogonal. Moreover, the manipulation of the cylindrical probe 110 is difficult because it presents an important inertia force due to its relatively heavy weight. Particularly when small objects having low rigidity such as sheet metal are to be measured, the inertia force of this bulky probe itself provokes torsion or deformation of the object to be measured, increasing its measurement error. Not only the manual operation of a coordinate measuring apparatus or a digitizer but also the teaching operation of a CNC three-dimensional measuring apparatus encounter such problems.